ICE with mir-opt-level=2: store_with_flags: invalid ScalarPair layout

[bytwise]

PR #66282 introduced an ICE in the following code with -Z mir-opt-level=2:

enum Src {
    Foo(u8),
    Bar,
}

enum Dst {
    Foo(u8),
}

fn main() {
    let e: Src = Src::Foo(0);
    let _: Dst = match e {
        Src::Foo(x) => Dst::Foo(x),
        Src::Bar => Dst::Foo(0),
    };
}

Compiler output

error: internal compiler error: src/librustc_codegen_ssa/mir/operand.rs:329: store_with_flags: invalid ScalarPair layout: TyLayout {
    ty: Dst,
    details: LayoutDetails {
        variants: Single {
            index: 0,
        },
        fields: Arbitrary {
            offsets: [
                Size {
                    raw: 0,
                },
            ],
            memory_index: [
                0,
            ],
        },
        abi: Scalar(
            Scalar {
                value: Int(
                    I8,
                    false,
                ),
                valid_range: 0..=255,
            },
        ),
        largest_niche: None,
        align: AbiAndPrefAlign {
            abi: Align {
                pow2: 0,
            },
            pref: Align {
                pow2: 3,
            },
        },
        size: Size {
            raw: 1,
        },
    },
}

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:892:9
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: rustc_driver::report_ice
   6: std::panicking::rust_panic_with_hook
   7: std::panicking::begin_panic
   8: rustc_errors::HandlerInner::bug
   9: rustc_errors::Handler::bug
  10: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  11: rustc::ty::context::tls::with_opt::{{closure}}
  12: rustc::ty::context::tls::with_opt
  13: rustc::util::bug::opt_span_bug_fmt
  14: rustc::util::bug::bug_fmt
  15: rustc_codegen_ssa::mir::operand::OperandValue<V>::store_with_flags
  16: rustc_codegen_ssa::mir::rvalue::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_rvalue
  17: rustc_codegen_ssa::mir::codegen_mir
  18: rustc_codegen_ssa::base::codegen_instance
  19: <rustc::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define
  20: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  21: rustc::dep_graph::graph::DepGraph::with_task
  22: rustc_codegen_llvm::base::compile_codegen_unit
  23: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  24: rustc_interface::passes::start_codegen::{{closure}}
  25: rustc_interface::passes::start_codegen
  26: rustc::ty::context::tls::enter_global
  27: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  28: rustc_interface::passes::create_global_ctxt::{{closure}}
  29: rustc_interface::passes::BoxedGlobalCtxt::enter
  30: rustc_interface::queries::Query<T>::compute
  31: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  32: rustc_interface::interface::run_compiler_in_existing_thread_pool
  33: std::thread::local::LocalKey<T>::with
  34: scoped_tls::ScopedKey<T>::set
  35: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.41.0-nightly (e87a205c2 2019-11-27) running on x86_64-apple-darwin

note: compiler flags: -Z mir-opt-level=2

query stack during panic:
end of query stack
error: aborting due to previous error

[scottmcm]

cc @Centril

with mir-opt-level=1:

    bb3: {
        StorageLive(_4);                 // bb3[0]: scope 1 at ./example.rs:13:18: 13:19
        _4 = ((_1 as Foo).0: u8);        // bb3[1]: scope 1 at ./example.rs:13:18: 13:19
        StorageLive(_5);                 // bb3[2]: scope 3 at ./example.rs:13:33: 13:34
        _5 = _4;                         // bb3[3]: scope 3 at ./example.rs:13:33: 13:34
        ((_2 as Foo).0: u8) = move _5;   // bb3[4]: scope 3 at ./example.rs:13:24: 13:35
        discriminant(_2) = 0;            // bb3[5]: scope 3 at ./example.rs:13:24: 13:35
        StorageDead(_5);                 // bb3[6]: scope 3 at ./example.rs:13:34: 13:35
        StorageDead(_4);                 // bb3[7]: scope 1 at ./example.rs:13:35: 13:36
        goto -> bb4;                     // bb3[8]: scope 1 at ./example.rs:12:18: 15:6
    }

with mir-opt-level=2:

    bb3: {
        _2 = move _1;                    // bb3[0]: scope 1 at ./example.rs:13:18: 13:19
        goto -> bb4;                     // bb3[1]: scope 1 at ./example.rs:12:18: 15:6
    }

Where the types are

    let _1: Src as UserTypeProjection { base: UserType(0), projs: [] }; // in scope 0 at ./example.rs:11:9: 11:10
    let mut _2: Dst;                     // in scope 0 at ./example.rs:12:18: 15:6